Process for refining lead alloys



Patented Apr. 26, 1938 UNITED STATES PROQESS FOR REFINING LEAD ALLOYS William Thomas Butcher, Ilford, England, assignor to Goodlass Wall & Lead Industries Limited, London, England No Drawing. Application July 22, 1937, Serial 12 Claims.

In the known process for removing tin from lead alloys which consists in effecting. preferential oxidation of the tin in the presence of lead chloride, the tin oxide separates out in the lead chloride layer and can be removed from the metal bath substantially uncontaminated with oxides of other metals.

I have found that metals can be removed from lead .alloys by oxidation in the presence of a flux consisting of lead fluoride and lead oxide at relatisvely low temperatures of the order of 450- 5 0 C.

Although lead fluoride melts at a temperature of about 850 C., it forms low melting point mixtures with lead oxide, thus enabling the extraction of metals from lead alloys to be very effectively carried out at a relatively low temperature.

These mixtures have the additional-advantage of being fumeless.

One example of such mixtures is the eutectic mixture of lead fluoride and lead oxide which melts at a temperature of about 490 C. although other mixtures within .a wide range of composition can be used. When increased fluidity of a flux is desired it can be obtained by an addition of lead sulphate, preferably in a quantity of 20-25%.

The method according to the invention can be used for the removal, from a molten mixture of metals containing lead as its principal constituent, of any metal having, under the conditions obtaining, a greater affinity for oxygen than has lead. The metals which we have removed by this method include tin, antimony, arsenic, nickel. cadmium, zinc, magnesium, aluminium, and the alkali and alkaline earth metals. The method is not, however, applicable to the removal from molten lead alloys of silver, bismuth, copper, tellurium or selenium.

An important aspect of the present invention is the completeness and rapidity with which metals can be removed from lead alloys and also the fact that small concentrations of metal in the alloy can be extracted with relatively high concentrations of the extracted metals in the flux. In one instance lead containing 0.07% antimony was refined to below 0.001% antimony by stirring with a lead fluoride-lead oxlde'flux for five minutes when the concentration of antimony in the flux was found to be 10%.

The invention has a number of important industrial applications. It may be used in the refining of silver lead. Thus metals such as tin, antimony, zinc, aluminium, arsenic and cadmium which may be present can all be removed In Great Britain August 8, 1936 I have found that in certain circumstances it may be advantageous to carry out the reaction in a nonoxidizing atmosphere, thus enabling the amount of oxygen taking part in the reaction to be controlled by the amount of metal oxide added.

The lead oxide which serves to oxidize the metal to be removed from the bath can be formed in situ by adding to the-bath a lead compound, such as lead nitrate or lead carbonate which will decompose at the temperature of the bath to give lead oxide, by blowing air into the bath, or by introducing air by stirring, or otherwise agitating the surface of the metal.

The following are;examples of how the invention can be carried'into practice:

In each of the examples given below, th'e lead alloy to be treated is melted in a kettle and heated to about 500 C., and the lead fluoride and the whole or part of the lead oxide are then added. This mixture melts and forms a fluid layer on top of the molten alloy. Stirring is efiected by the usual means and the balance, if any, of the lead oxide added. The metal to be removed from the alloy is taken up by the flux and is replaced in the alloy by lead reduced to the elementary state from the lead oxide used.

In some cases it is preferable to make more than one flux addition and this is specially so where preferential removal of a certain metal from the bath is aimed at...

In one case 75 parts of an alloy containing,

5.8% antimony and 94.2% lead were treated. Two parts by weight of lead fluoride mixed with three parts lead oxide were added to the molten alloy at a temperature of 500 C. On stirring, a fluid fiux was formed at once. After a few minutes a slight thickening of the flux was observedz-The addition of a-further three parts of lead oxide restored the liquid condition of the flux. Further additions of lead oxide were made from time to time to restore fluidity of the flux as antimony was removed from the alloy until altogether 15 parts of litharge had been absorbed. The flux was still very fluid and the temperature of the bath of metal 490 C.

An analysis of the flux showed that'it contained 34.0% antimony, while the alloy contained 1.17% antimony.

The flux weighed 10 parts and the alloy '79 parts. The increase in the final weight of the alloy is due to the replacement of antimony by lead. At the same time the flux shows a loss of weight.

In the same way by further treatment with fresh lead fluoride-lead oxide mixture the remaining antimony in the alloy can be taken out leaving substantially pure lead.

It has also been found that lead drosses and residues such as are produced in the melting of lead and lead alloys may be used in the place of pure lead oxide. These drosses may contain lead and/or other metals in the metallic form or as oxides. When such material is in contact with the fluid flux the metallic portion melts and joins the metal bath leaving the oxide in the flux.

The method may be applied to the removal of antimony from lead antimony alloys containing such elements as copper, and tellurium, when'it is found that the last two elements are substantially unaii'ected and remain in the alloy;

The method according to the invention is readily applicable to the removal of zinc from lead and lead alloys, as for example in the residual zinc in the Parkes desilverizing process.

In one case 33 tons of desilverized bullion containing 0.5% of residual'zinc was treated in a pot at. 550 C., with 1 cwts. of lead fluoride and 5 /2 cwts. olfdead oxide and the whole stirred for 95 minutes. A dry flu'x containing 32.6% zinc was then removed and the metal was found to be substantially free of zinc. Thisreaction also finds ready application in the removal of zinc from lead zinc alloys such as are produced as a bye-product by galvanizers.

III

' The following is an example of the applicationof the invention to the removal of tin from lead alloys.

About 35 tons of a lead alloy containing 0.6%

tin, 0.12% antimony and 0.04% copper were melted in a pot and heated to 550 C. 250 lbs: of lead fluoride and, 1284 lbs. of lead oxide dross containing approximately 80% metallic lead and 20% lead oxide were stirred into the metal for 1 /3 hours and the flux removed. Upon analysis the flux was found to contain 91% of the weight of tin in the original metal and 2% of the antimony. The concentration .of tin in the flux was 25.7% and of antimony 0.9%. 0n treatment with a further quantity of lead fluoride lead oxide mixture the remainder of the tin and antimony was removed from the metal in a flux which was found on analysis to contain 0.8%tin and 3.0% anti-'A mony. This aifords an example of the preferential removal of tin from leadalloys containing antimony. The copper remained unchanged in the metal at 0.04%. In another-example 40 tons of lead containing 3.3% tin" and 1.4% antimony were treated with 10 cwts. of lead fluoride and 27 cwts. of a lead oxide dross containing about 40% metallic lead and .lead oxide. After stirring for 18 hours the flux was removed and contained 46.0% tin and 1.3% antimony. The

metal on analysis was found to contain 1.1% tin and 1.5% antimony. Subsequent treatment with lead oxide lead fluoride mixtures removed the whole of the tin from the metal.

My experiments have shown that arsenic, nickel, cadmium, aluminium, magnesium, lithie um; sodium, potassium, strontium and calcium can be removed from lead alloys'in a similar manner using the lead fluoride lead oxide flux.

After completion of the extraction operation,

the lead fluoride flux may be smelted direct with carbon and lime, the fluorine passing into the slag as calcium fluoride.

Alternatively the flux may be mixed with a pro-/ portion of carbon sufficient only to reduce a part of the metal oxides and the residual flux now enriched in lead fluoride can be used again. This method is applicable for instance in the case of a flux containing antimony oxide. For example 7 cwts. of a lead fluoride antimony oxide flux containing 32% antimony were stirred on top of a bath of pure lead for 3 hours and 38 lbs. of anthracite weregradually added. 42% of the contained antimony with only 2% of the contained lead in the flux was reduced to metal. The resulting flux was used again for further antimony extraction.

In this way alloys of lead and antimony, for example, may be prepared in a substantially pure state and the residual flux now enriched in lead fluoride contentbe used again for the extraction of further amounts of antimony.

What I claim as my invention and desire to I secure by Letters Patent is:-.- a.

1. A process for removing metals from lead alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture of lead fluoride and lead oxide, thereby effecting effecting oxidation of the said metal and replace- I ment thereof 'in the alloy by lead, and removing the oxide of the said metal so formed by separating the flux from the alloy.

3. A process for removing metals from lead alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture of lead fluoride and lead oxide, the lead oxide being formed in situ, thereby eflecting oxidation of the said metal and replacement thereof in the alloy by lead, and removing the oxide of the said metal so formed by separating the flux from the alloy.

4. A .process for removing metals from lead .alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture of lead fluoride and lead oxide, thereby effecting oxidation of the said metal and replacement thereof in the alloy by lead, adding further lead oxide to the flux as it thickens due to the concentration therein of oxide of the said metal, and

finally separating the flux from 'the alloy.

5. A process for removing metals from lead alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture of lead fluoride, lead sulphate and lead oxide,

thereby effecting oxidation of the said metal-and replacement thereof in the alloy by lead, and removing the oxide of the said metal so formed by separating the flux from the alloy.

6. A process for removing metals from lead alloys according to claim 1, in which the lead oxide is introduced into the flux in lead dross or residue.

7. A process for removing metals fromlead alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture the form of 9.

' of lead fluoride and lead oxide, thereby effecting oxidation of the said metal and replacement thereof in the alloy by lead, removingthe oxide of the said metal so formed by separating the flux from the alloy, and finally treating the flux with a quantity of carbon suflicientto reduce to metal a part only of the metallic oxides contained in the flux, so rendering the flux suitable for re-use. I

8. A process for removing metals from lead alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture of leadfluoride and lead oxide, thereby effecting oxidation of the said metal and replacement thereof in the alloy by lead, removing the oxide of the said metal so formed by separating the flux from the alloy, and finally smelting the flux direct toreduce the contained oxides to metal.

9. A process for removing antimony from lead alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture of lead fluoride and lead oxide, thereby effecting oxidation of the antimony and replacement thereof in the alloy by lead, and removing the antimony oxide so formed by separating the flux from the alloy.

10. A process for removing 'zinc from lead alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture of lead fluoride and lead oxide, thereby efiecting oxidation of the zinc and replacement. thereof in the alloy by lead, and removing the zinc oxide so formed byseparating the flux from the alloy.

11. A process for removing tin from lead alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture of lead fluoride and lead oxide,-therebyefiecting oxidation of the tin and replacement thereof in the alloy by lead, and removing the tin oxide so formed by separating the flux from the alloy.

, 12. A process for removing a metal of the group consisting of antimony, zinc and tin from lead alloys, which comprises the steps of treating the molten alloy with a flux consisting of a mixture oflead fluoride and lead oxide, thereby effecting oxidation of the said metal, and removing the oxide so formed by separating the flux from the alloy.

THOMAS BUTCHER. 

